This invention relates to a process for preparing microparticles comprising a solid organopolysiloxane. More particularly, this invention relates to the preparation of microspheres and microcapsules using a two-part siloxane composition wherein one siloxane polymer contains amine functionality and is capable of reacting with the other siloxane polymer, which contains acryloxy, methacryloxy or acrylamide functionality (hereinafter acryl-functional siloxane). The reaction, which leads to a cured elastomer or resin, takes place via a Michael-type addition between the amine-functional siloxane and the acryl-functional siloxane.
The reaction between amine-functional and alpha-beta olefinically unsaturated compounds is well known in the art and is often referred to as a Michael addition. Pike et al., in U.S. Pat. No. 3,033,815, disclose the preparation of organosilicon compounds containing a substituted amino group attached to at least one silicon atom thereof through an alkylene linkage of at least three carbon atoms. This is accomplished by reacting an aminoalkyl silicon compound with a variety of alpha-beta olefinically unsaturated organic compounds. The reactive aminoalkylene group can be attached to a silane or siloxane structure. The compounds taught by Pike et al. are said to find use as sizes for fibrous materials, modifiers for polydimethylsiloxane oils and gums, adhesives and flocculation agents.
In Pepe et al., U.S. Pat. No. 4,122,074 polyester aminoalkylalkoxysilanes are provided by effecting the Michael addition reaction of an unsaturated conjugated polyester with an aminoalkylalkoxysilane. These polymers are said to have utility as adhesion promoters between siliceous materials and organic resins and can further act as a size, or protective coating, when the siliceous material is glass cloth or fiber.
Pepe U.S. Pat. No. 4,209,455, discloses aminoorganosilicon acylamino compounds which are prepared by a Michael addition of an amino-functional alkoxysilane to an olefinic carboxylate ester to form the corresponding amino-carboxylic acid ester of the silane. This amino-carboxylic acid ester-silane is then amidated with either a primary amino silicon compound or a primary organoamine to produce the desired aminoorganosilicon acylamino compound. Alkoxy groups on the silicon of these final compounds may be hydrolyzed and condensed, with or without other conventional organosilanes, to form polymeric siloxanes having the respective amine functionality attached thereto. The silanes and siloxane compositions thus produced are said to be useful as sizes for fibrous materials (i.e., as coupling agents) and as additives for hydroxyl containing organic thermoplastic polymer compositions.
Lee U.S. Pat. No. 4,429,082, disclose film-forming resins produced by the Michael addition of an amine-functional silane or amine-functional siloxane to a condensation product which contains at least 2 amine reactive acrylyl residues per molecule. Examples of useful condensation products (backbone polymer) include unsaturated polyester polyol polyacrylate, alkydpolyol polyacrylate and polyurethanepolyol polyacrylate. The silanes employed in the Michael addition contain alkoxy groups which are retained after the addition and allow the products of this invention to be cured with the aid of organometallic catalysts. The resins so produced are said to be useful film-forming components in coating compositions which can be cured at low temperatures and preferably include a low organic solvent content.
Acryl-functional silicone compounds are disclosed by Lee and Lutz in copending application Ser. No. 816,477, filed Jan. 6, 1986 now U.S. Pat. No. 4,697,026, and assigned to the assignee of this invention. In this case, acryl silane (or silicone) compounds are made by reacting an amine-functional silane (or amine-functional silicone) with a di- or multi-functional acryl compound by a Michael addition reaction. These acryl silane (silicone) compounds are said to be purer than those formed by other routes since no catalyst is used and no by-products are formed. The compounds are further said to have utility as adhesion promoters (silanes) and as coatings compositions which can be cured by ultraviolet radiation (silicones).
The Michael addition reaction has also been employed in the heat cure of electrocoating compositions. McGinniss, in U.S. Pat. No. 3,975,251, teaches coating a cathode substrate with a polymer having pendant amine groups and a cross-linking agent having alpha-beta-ethylenically unsaturated carbonyl groups. The coating process employs an aqueous dispersion of the polymer and cross-linking agent wherein the amine groups are first protonated with an acid to render the polymer dispersible prior to electrodeposition. Upon electrodeposition, the amine groups become deprotonated and then cross-link with the alpha-beta-ethylenically unsaturated carbonyls upon heating to form a fully cured coating on the substrate.
Buchwalter et al., in U.S. Pat. No. 4,198,331, teach a curable resinous composition comprising a polyamine resin containing amino groups, which are stable under the curing conditions, and a Michael adduct of an organic material containing alpha, beta-ethylenically unsaturated moieties in conjugation with carbonyl moieties reacted with amines, which is unstable under curing conditions. When these compositions are heated to curing temperatures, it is believed the adduct decomposes releasing amine which volatilizes, generating alpha, beta-ethylenically unsaturated carbonyls which cure via a Michael addition reaction with non-volatile amines present in the composition.
The use of silicones in the preparation of microcapsules is known in the art. Ziemelis, in U.S. Pat. No. 4,370,160, hereby incorporated by reference, teaches a process for preparing microspheres of solid organopolysiloxane or microcapsules which consist essentially of an internal material dispersed throughout a solid organopolysiloxane. In brief, this process comprises irradiating a dispersion of discrete entities with ultraviolet light. The discrete entities are dispersed in a UV-transparet fluid continuous phase and are sphere-like particles of a UV-curable, liquid organopolysiloxane composition which is immiscible with the continuous phase, or such a liquid organopolysiloxane composition containing a material to be encapsulated. The microparticles may be elatomeric or resinous and are useful as filler particles and time-release capsules. The liquid organopolysiloxane composition, in turn, consists essentially of a mixture of an organopolysiloxane having at least two vinyl (or butenylene) radicals per molecule and an organopolysiloxane having at least two mercaptoalkyl (or silicon-bonded hydrogen) radicals per molecule.
The process taught by Ziemelis, although quite useful for UV-transparent materials, can find little utility when the continuous phase itself is UV-opaque. In such an instance, irradiation would be ineffective, save in very thin sections. Furthermore, should the material to be encapsulated contain any component which can "poison" or retard the UV-care, this process would again be of little value.
These references do not suggest the use of the Michael addition reaction as a method for cross-linking silicone polymers and copolymers, particularly as applied to the formation of silicone microshperes and microcapsules as described in the instant invention.